Atomic Number: Z:
The atomic number of an atom, denoted by Z, is the number of
protons present inside the nucleus of an atom.
Number of Electrons:
For a neutral atom, the number of electrons always equal the
number of protons (or atomic number; Z).
Mass Number: A:
The mass number of an atom, denoted by A, is the sum of the
number of protons and the number of neutrons present inside the nucleus of an
atom.
(Note: The mass of electrons is almost negligible as relative to
proton and/or neutron.)
Number of Neutrons:
For any case, the number of neutrons in an atom can be
calculated as:
Number of Neutrons = Mass Number - Atomic Number = A – Z
Chemical Representation of an Atom:
This is also how the elements are shown in the Periodic
Table.
Table 4: First 20
elements of the Periodic Table:
Name
|
Symbol
|
Mass Number : A
|
Number of Protons /
Atomic Number : Z
|
Number of Neutrons
= A - Z |
Number of Electrons
|
Hydrogen
|
H
|
1
|
1
|
0
|
1
|
Helium
|
He
|
4
|
2
|
2
|
2
|
Lithium
|
Li
|
7
|
3
|
4
|
3
|
Beryllium
|
Be
|
8
|
4
|
4
|
4
|
Boron
|
B
|
10
|
5
|
5
|
5
|
Carbon
|
C
|
12
|
6
|
6
|
6
|
Nitrogen
|
Ni
|
14
|
7
|
7
|
7
|
Oxygen
|
O
|
16
|
8
|
8
|
8
|
Fluorine
|
F
|
18
|
9
|
9
|
9
|
Neon
|
Ne
|
20
|
10
|
10
|
10
|
Sodium
|
Na
|
23
|
11
|
12
|
11
|
Magnesium
|
Mg
|
24
|
12
|
12
|
12
|
Aluminium
|
Al
|
27
|
13
|
14
|
13
|
Silicon
|
Si
|
28
|
14
|
14
|
14
|
Phosphorus
|
P
|
30
|
15
|
15
|
15
|
Sulphur
|
S
|
32
|
16
|
16
|
16
|
Chlorine
|
Cl
|
35
|
17
|
18
|
17
|
Argon
|
Ar
|
36
|
18
|
18
|
18
|
Potassium
|
K
|
38
|
19
|
19
|
19
|
Calcium
|
Ca
|
40
|
20
|
20
|
20
|
